Conversion of carbon dioxide to organic products

ABSTRACT

The invention relates to various embodiments of an environmentally beneficial method for reducing carbon dioxide. The methods in accordance with the invention include electrochemically or photoelectrochemically reducing the carbon dioxide in a divided electrochemical cell that includes an anode, e.g., an inert metal counterelectrode, in one cell compartment and a metal or p-type semiconductor cathode electrode in another cell compartment that also contains an aqueous solution of an electrolyte and a catalyst of one or more substituted or unsubstituted aromatic amines to produce therein a reduced organic product.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/206,286, filed on Jan. 29, 2009, which is hereby incorporated byreference.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with United States government support fromNational Science Foundation Grant No. CHE-0616475. The United StatesGovernment has certain rights in this invention.

INTRODUCTION

In recent years, high levels of atmospheric carbon dioxide (CO₂),emitted, for example, from industry, fossil fuel combustion andutilities, have been linked to global climate change. A greenhouseeffect attributed to carbon dioxide is indicated as one cause of thewarming phenomenon of the earth. Many responsible sources contend thatthe condition of the earth's atmosphere is such that, to avoid thepredicted dire consequences of global warming effects, removal of aportion of the existing, as well as new, quantities of carbon dioxidefrom the atmosphere is needed.

Various options for carbon dioxide reduction have been proposed. Inaddition to energy conservation, carbon capture and storage, the processof separating CO₂ from emission sources and transporting it to a storagelocation for long-term (indefinite) isolation, and carbon sequestration,the process of permanently storing CO₂ underground, have garnered themost attention to date. However, these technologies face significantchallenges and are presently far from being cost effective. In addition,sequestration has raised serious environmental concern, legal andregulatory issues due to the unknown ramifications of permanentlystoring CO₂ underground.

A significant issue with the removal of carbon dioxide from theatmosphere is the very large energy input to overcome the entropicenergies associated with isolating and concentrating a diffuse gas. Asnoted, current strategies for removal of carbon dioxide from theatmosphere are either inefficient, cost prohibitive, or produce toxicby-products such as chlorine. To lower global carbon dioxide levels andreduce new carbon dioxide emissions, it remains critical to developeconomically feasible processes to remove vast quantities of carbondioxide from the atmosphere or gas streams.

BRIEF DESCRIPTION

In accordance with embodiments of the invention, an electrocatalyticsystem is provided that allows carbon dioxide to be converted at verymodest overpotentials to highly reduced species in aqueous solution. Inother words, carbon-carbon and/or carbon-hydrogen bonds are formed inaqueous solution under very mild condition utilizing a minimum ofenergy. In some embodiments, the required energy input may be generatedfrom an alternative energy source or directly using visible lightdepending on how the system is implemented.

In embodiments of the invention, the reduction of carbon dioxide issuitably catalyzed by aromatic heterocyclic amines, e.g., pyridinium,imidazole and their substituted derivatives. These simple organiccompounds have been found to be effective and stable homogenouselectrocatalysts and photoelectrocatalysts for the aqueous multipleelectron, multiple proton reduction of carbon dioxide to organicproducts such as formic acid, formaldehyde, and methanol. For productionof methanol, the reduction of carbon dioxide proceeds along 6 e⁻transfer pathway. High faradaic yields for the reduced products havebeen found in both electrochemical and photoelectrochemical systems atlow reaction overpotentials.

It has previously been thought that metal-derived multi-electrontransfer was necessary to achieve highly reduced products such asmethanol. Surprisingly, the simple aromatic heterocyclic amine moleculesin accordance with embodiments of the invention are capable of producingmany different chemical species on route to methanol through multipleelectron transfers instead of metal-based multi-electron transfer.

The invention thus relates to various embodiments of environmentallybeneficial methods for reducing carbon dioxide. The methods inaccordance with the invention include electrochemically orphotoelectrochemically reducing the carbon dioxide in an aqueous,electrolyte-supported divided electrochemical cell that includes ananode, e.g., an inert metal counterelectrode, in one cell compartmentand a metal or p-type semiconductor working cathode electrode in anothercell compartment that contains a catalyst of one or more substituted orunsubstituted aromatic heterocyclic amines to produce a reduced organicproduct. CO₂ is continuously bubbled through the cathode electrolytesolution to saturate the solution.

For electrochemical reduction, the electrode may be chosen from anysuitable metal electrode, such as Au, Ag, Zn, Pd, Ga, Hg, In, Cd, Ti andPt. Pt and hydrogenated Pd have been found to be especially suitable.For photoelectrochemical reduction, the electrode may suitably be chosenfrom p-type semiconductors such as p-GaAs, p-GaP, p-InN, p-InP, p-CdTe,p-GaInP₂ and p-Si.

The catalyst for conversion of carbon dioxide electrochemically orphotoelectrochemically may be selected from any substituted orunsubstituted aromatic heterocyclic amine. Suitable amines areheterocycles which are 5- or 6-member rings with at least one ringnitrogen. For example, pyridine, imidazole and their substitutedderivatives have been found to be especially suitable as catalysts foreither the electrochemical reduction or the photoelectrochemicalreduction. It is also envisioned the other aromatic amine, e.g.,quinolines, are also effective electrocatalysts.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood and appreciated by reference tothe detailed description of specific embodiments presented herein inconjunction with the accompanying drawings of which:

FIG. 1 is a flow chart for the electrochemical reduction of CO₂ inaccordance with embodiments of the invention; and

FIG. 2 is a flow chart for the photoelectrochemical reduction of CO₂ inaccordance with embodiments of the invention.

DETAILED DESCRIPTION

Embodiments of the present invention relate to the simple, efficient,and economical conversion of carbon dioxide to reduced organic products,such as methanol, formic acid and formaldehyde.

It has been previously known that carbon dioxide can be photochemicallyor electrochemically reduced to formic acid with formaldehyde andmethanol being formed in only smaller amounts. Catalytic hydrogenationof carbon dioxide using heterogeneous catalysts is also known to providemethanol together with water as well as formic acid and formaldehyde.Also known is the reduction of carbon dioxide to methanol with complexmetal hydrides, such as lithium aluminum hydride, a process which isextremely costly, and therefore, not suited for the bulk production ofmethanol. Such known current processes are highly energy-consuming, andare not efficient ways for a high yield, economical conversion of carbondioxide to organic products, e.g., methanol.

On the other hand, the use of processes for converting carbon dioxide toreduced organic products in accordance with embodiments of the inventionhas the potential to lead to a significant reduction of carbon dioxide,a major greenhouse gas, in the atmosphere, thus to mitigation of globalwarming. Moreover, the present invention advantageously producesmethanol and related products without the need of adding extrareactants, such as a hydrogen source. The resultant product mixturerequires little in the way of further treatment. For example, aresultant 1 M methanol solution may be used directly in a fuel cell. Forother uses, simple removal of the electrolyte salt and water are readilyaccomplished.

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application ofthe details of the structure or the function of the invention set forthin the following description or illustrated in the appended figures ofthe drawing. The invention is capable of other embodiments and of beingpracticed or carried out in various ways. Also, it is to be understoodthat the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of termssuch as “including,” “comprising,” or “having” and variations thereofherein is meant to encompass the item listed thereafter and equivalentsthereof as well as additional items. Further, unless otherwise noted,technical terms are used according to conventional usage.

Further, unless otherwise noted, technical terms are used according toconventional usage. Definitions of standard chemistry terms may be foundin reference works, such as Carey and Sundberg “ADVANCED ORGANICCHEMISTRY 4^(th) ED.” Vols. A (2000) and B (2001), Plenum Press, NewYork. Unless otherwise indicated, conventional methods of massspectroscopy, NMR, spectrophotometry, and gas chromatography, within theskill of the art are employed. The nomenclature employed in connectionwith, and the laboratory procedures and techniques of, electrochemistry,analytical chemistry, and synthetic organic chemistry described hereinare generally those known in the art. However, as used herein, thefollowing definitions may be useful in aiding the skilled practitionerin understanding the invention.

An “alkyl” group refers to an aliphatic hydrocarbon group. The alkylmoiety may be a “saturated alkyl” group (i.e., no alkene or alkynemoieties), or an “unsaturated alkyl” moiety, which means that itcontains at least one alkene or alkyne moiety. The alkyl moiety, whethersaturated or unsaturated, may be branched, straight chain, or cyclic.Depending on the structure, an alkyl group can be a monoradical or adiradical (i.e., an alkylene group). As used herein, is the designationC₁-C_(X), which includes C₁-C₂, C₁-C₃, C₁-C₄ . . . C₁-C₁₀ . . .C₁-C_(X).

The “alkyl” moiety may have 1 to 30 carbon atoms (whenever it appearsherein, a numerical range such as “1 to 30” refers to each integer inthe given range; e.g., “1 to 30 carbon atoms” means that the alkyl groupmay have 1 carbon atom, 2 carbon atoms, 3 carbon atoms, etc., up to andincluding 30 carbon atoms, although the present definition also coversthe occurrence of the term “alkyl” where no numerical range isdesignated). A “lower alkyl” moiety may have 1 to 10 carbons. Forexample, the lower alkyl group of the compounds described herein may bedesignated as “C₁-C₁₀ alkyl” or similar designations. By way of exampleonly, “C₁-C₁₀ alkyl” includes C₁-C₂ alkyl, C₁-C₃ alkyl, C₁-C₄ . . .C₁-C₁₀ alkyl. Typical lower alkyl groups include, but are in no waylimited to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tertiarybutyl, pentyl, hexyl, ethenyl, propenyl, butenyl, cyclopropyl,cyclobutyl, cyclopentyl, cyclohexyl, heptyl, octyl, nonyl, and decyl.Lower alkyl groups can be substituted or unsubstituted.

The term “aromatic” refers to a planar ring having a delocalizedπ-electron system containing 4n+2 π-electrons, where n is an integer.Aromatic rings can be formed by five, six, seven, eight, nine, or morethan nine atoms. Aromatics can be optionally substituted. The term“aromatic” includes both carbocyclic aryl (e.g., phenyl) andheterocyclic aryl (or “heteroaryl” or “heteroaromatic”) groups (e.g.,pyridine). The term also includes monocyclic or fused-ring polycyclic(i.e., rings which share adjacent pairs of carbon atoms) groups.

As used herein, the term “aryl” refers to an aromatic ring wherein eachof the atoms forming the ring is a carbon atom. Aryl rings can be formedby five, six, seven, eight, nine, or more than nine carbon atoms. Arylgroups can be optionally substituted. Examples of aryl groups include,but are not limited to phenyl, naphthalenyl, phenanthrenyl, anthracenyl,fluorenyl, and indenyl. Depending on the structure, an aryl group can bea monoradical or a diradical (i.e., an arylene group).

As used herein, the term “ring” refers to any covalently closedstructure. Rings include, for example, carbocycles (e.g., aryls andcycloalkyls), heterocycles (e.g., heteroaryls and non-aromaticheterocycles), aromatics (e.g. aryls and heteroaryls), and non-aromatics(e.g., cycloalkyls and non-aromatic heterocycles). Rings can beoptionally substituted. Rings can also form part of a ring system.

As used herein, the term “ring system” refers to two or more rings,wherein two or more of the rings are fused. The term “fused” refers tostructures in which two or more rings share one or more bonds.

The terms “heteroaryl,” “heteroaromatic” or “aromatic heterocyclic”refers to an aryl group that includes one or more ring heteroatomsselected from nitrogen, oxygen and sulfur. An N-containing“heteroaromatic” or “heteroaryl” moiety refers to an aromatic group inwhich at least one of the skeletal atoms of the ring is a nitrogen atom.The polycyclic heteroaryl group may be fused or non-fused. Illustrativeexamples of N-containing aromatic heterocyclic groups include thefollowing moieties:

and the like. Depending on the structure, a heteroaryl group can be amonoradical or a diradical (i.e., a heteroarylene group).

The term “membered ring” can embrace any cyclic structure. The term“membered” is meant to denote the number of skeletal atoms thatconstitute the ring. Thus, for example, cyclohexyl, pyridine, and pyranare 6-membered rings and cyclopentyl and pyrrole, are 5-membered rings.

The term “moiety” refers to a specific segment or functional group of amolecule. Chemical moieties are often recognized chemical entitiesembedded in or appended to a molecule.

As used herein, the substituent “R” appearing by itself and without anumber designation refers to a optional substituent as defined incertain formulas herein.

In the following description of methods in accordance with embodimentsof the invention, process steps are carried out at temperatures of 10°C. to 50° C. and pressures of 1 to 10 atmospheres unless otherwisespecified. It also is specifically understood that any numerical rangerecited herein includes all values from the lower value to the uppervalue, e.g., all possible combinations of numerical values between thelowest value and the highest value enumerated are to be considered to beexpressly stated in the application. For example, if a concentrationrange or beneficial effect range is stated as 1% to 50%, it is intendedthat values such as 2% to 40%, 10% to 30%, or 1% to 3%, etc., areexpressly enumerated in this specification. These are only examples ofwhat is specifically intended.

Further, no admission is made that any reference, including any patentor patent document, cited in this specification constitutes prior art.In particular, it will be understood that unless otherwise stated,reference to any document herein does not constitute an admission thatany of these documents forms part of the common general knowledge in theart in the United States or in any other country. Any discussion of thereferences states what their authors assert, and applicant reserves theright to challenge the accuracy and pertinence of any of the documentscited herein.

It has now been found that the use of electrochemical orphotoelectrochemical reduction of carbon dioxide (CO₂), tailored withcertain electrocatalysts, produces methanol and related products in ahigh yield of about 60 to about 100%, based on the amount of CO₂,suitably about 75 to 90%, and more suitably about 85 to 95%. At anelectric potential of about −0.09 to −0.5 V with respect to a standardcalomel electrode (SCE), methanol can be produced with good faradaicefficiency at the cathode.

The overall reaction for the reduction of CO₂ is represented as:CO₂+2H₂O→CH₃OH+3/2O₂For the 6 e-reduction, the reactions at the cathode and anode are:CO₂+6H⁺6e ⁻→CH₃OH+H₂O(cathode)3H₂O→3/2O₂+6H⁺+6e ⁻(anode)

FIGS. 1 and 2 illustrate the general method in accordance withembodiments of the invention for catalyzed electrochemical andphotoelectrochemical conversion of CO₂ to reduced organic products. Thereduction of CO₂ is suitably achieved in an efficient manner in adivided electrochemical or photoelectrochemical cell in which a firstcompartment contains an anode which is an inert counterelectrode, and asecond compartment containing a working cathode electrode and one ormore substituted or unsubstituted aromatic heterocyclic amines. Thecompartments are separated by a porous glass frit or other ionconducting bridge. Both compartments contain an aqueous solution of anelectrolyte such as KCl. CO₂ is continuously bubbled through thecathodic electrolyte solution to saturate the solution.

In the working electrode compartment, carbon dioxide is continuouslybubbled through the solution. In one embodiment, if the workingelectrode is a metal, then an external bias is impressed across the cellsuch that the potential of the working electrode is held constant, e.g.,between −0.5 V to −0.9 V v. SCE. In another embodiment, if the workingelectrode is a p-type semiconductor, the electrode is suitablyilluminated with light of energy equal to or greater than the bandgap ofthe semiconductor during the electrolysis, and either no external sourceof electrical energy is required or a modest bias of about 500 mV isapplied. The working electrode potential is held constant, e.g., between−0.5 to +0.2 V v. SCE. The electrical energy for the electrochemicalreduction of carbon dioxide can come from a conventional energy source,including nuclear and alternatives (hydroelectric, wind, solar power,geothermal, etc.), from a solar cell or other non-fossil fuel source ofelectricity, provided that the electrical source supply at least 1.6 Vacross the cell, although this minimum value may be adjusted dependingon the internal resistance of the cell employed.

Advantageously, the carbon dioxide used in the embodiments of theinvention can be obtained from any sources, e.g., an exhaust stream fromfossil-fuel burning power or industrial plants, from geothermal ornatural gas wells or the atmosphere itself. Most suitably, however,carbon dioxide is obtained from concentrated point sources of itsgeneration prior to its release into the atmosphere. For example, highconcentration carbon dioxide sources are those frequently accompanyingnatural gas in amounts of 5 to 50%, those from flue gases of fossil fuel(coal, natural gas, oil, etc.) burning power plants, and nearly pure CO₂exhaust of cement factories and from fermenters used for industrialfermentation of ethanol. Certain geothermal steams also containssignificant amounts of CO₂. In other words, CO₂ emissions from variedindustries, including geothermal wells, can be captured on-site.Separation of CO₂ from such exhausts is well-developed. Thus, thecapture and use of existing atmospheric CO₂ in accordance withembodiments of the invention allows CO₂ to be a renewable and unlimitedsource of carbon.

For electrochemical conversion, CO₂ is readily reduced in the aqueousmedium with a metal electrode, such as a Pt and hydrogenated Pdelectrode, although other metal electrodes, e.g., Au, Ag, Zn, Ga, Hg,In, Cd and Ti may also be effective. Faradaic efficiencies have beenfound to be high, reaching about 100%.

For photoelectrochemical conversion, CO₂ is readily reduced with ap-type semiconductor electrode, such as p-GaP, p-GaAs, p-InP, p-InN,p-WSe₂, p-CdTe, p-GaInP₂ and p-Si.

In embodiments of the invention, theelectrochemical/photoelectrochemical reduction of CO₂ utilizes one ormore substituted or unsubstituted aromatic heterocyclic amines ashomogeneous catalysts in aqueous solution. Aromatic heterocyclic aminesinclude, for example, unsubstituted and substituted pyridines, pyroles,imidazoles and benzimidazoles. Substituted pyridines and imidazoles mayinclude mono- and disubstituted pyridines and imidazoles. For example,suitable catalysts may include straight chain or branched chain loweralkyl (e.g., C₁-C₁₀) mono- and disubstituted compounds such as2-methylpyridine, 4-tertbutyl pyridine, 2,6-dimethylpyridine(2,6-lutidine); bipyridines, such as 4,4′-bipyridine; amino-substitutedpyridines, such as 4-dimethylamino pyridine; and hydroxyl-substitutedpyridines, e.g., 4-hydroxy-pyridine, and substituted or unsubstitutedquinoline or isoquinolines. Catalysts may also suitably includesubstituted or unsubstituted dinitrogen heterocyclic amines such apyrazine, pyridazine and pyrimidine.

In some embodiments, the aromatic heterocyclic amine catalysts may berepresented by formula 1:

wherein the ring structure W is an aromatic 5- or 6-member heterocylicring with at least one ring nitrogen and is optionally substituted atone or more ring positions other than nitrogen with R, andwherein L is C or N, R¹ is H, R² is H if L is N or R² is R if L is C,and R is an optional substitutent on any ring carbon and isindependently selected from H, straight chain or branched chain loweralkyl, hydroxyl, amino, pyridyl, or two R's taken together with the ringcarbons to which they are bonded are a fused six-member aryl ring, andn=0 to 4.

In some embodiments, the substituted or unsubstituted aromatic 5- or6-member heterocyclic amines may be represented by the followingformulas (2), (3) or (4). For example, a catalyst in accordance withembodiments of the invention which is a 6-member heterocycline ringhaving one nitrogen in the ring is represented by formula (2):

wherein R³ is H; R⁴, R⁵, R⁷ and R⁸ are independently H, straight chainor branched chain lower alkyl, hydroxyl, amino, or taken together withthe ring carbons to which they are bonded are a fused six-member arylring, and R⁶ is H, straight chain or branched chain lower alkyl,hydroxyl, amino or pyridyl.

A catalyst in accordance with embodiments of the invention which is a6-member heterocyclic amine having two nitrogen in the ring isrepresented by formula (3):

wherein one of L¹, L² and L³ is N, while the other L's are C, R⁹ is H,if L¹ is N, then R¹⁹ is H, if L² is N, then R¹¹ is H, and if L³ is N,then R¹² is H; and if L¹, L² or L³ is C, then R¹⁰, R¹¹, R¹², R¹³ and R¹⁴are independently selected from straight chain or branched chain loweralkyl, hydroxyl, amino, or pyridyl.

A catalyst in accordance with embodiments of the invention which is a5-member heterocyclic amine having one or two nitrogen in the ring isrepresented by formula (4):

wherein L⁵ is N or C, R¹⁵ is H, R¹⁶ is H if L⁵ is N, or if L⁵ is C, R¹⁶,R¹⁷, R¹⁸, and R¹⁹ are independently selected from straight chain orbranched chain lower alkyl, hydroxyl, amino, or pyridyl, or R¹⁷ and R¹⁸taken together with the ring carbons to which they are bonded are afused six-member aryl ring.

Suitably, the concentration of aromatic heterocyclic amine catalysts isabout 1-mM to 1 M. The electrolyte is suitably a salt, such as KCl orNaNO₃, at a concentration of about 0.5M. The pH of the solution ismaintained at about pH 3-6, suitably about 4.7-5.6.

At metal electrodes, formic acid and formaldehyde were found to beintermediate products along the pathway to the 6 e⁻ reduced product ofmethanol, with an aromatic amine radical, e.g., the pyridinium radical,playing a role in the reduction of both intermediate products. It hasbeen found, however, that these intermediate products can also be thefinal products of the reduction of CO₂ at metal or p-type semiconductorelectrodes, depending on the particular catalyst used. Other C—C coupleproducts are also possible. For example, reduction of CO₂ can suitablyyield formaldehyde, formic acid, glyoxal, methanol, isopropanol, orethanol, depending on the particular aromatic heterocyclic amine used asthe catalyst. In other words, in accordance with the invention, theproducts of the reduction of CO₂ are substitution-sensitive. As such,the products can be selectively produced. For example, use of4,4′-bipyridine as the catalyst can produce methanol and/or 2-propanol;lutidines and amino-substituted pyridines can produce 2-propanol;hydroxy-pyridine can produce formic acid; imidazoles can producemethanol or formic acid depending on conditions.

The effective electrochemical/photoelectrochemical reduction of carbondioxide disclosed herein provides new methods of producing methanol andother related products in an improved, efficient, and environmentallybeneficial way, while mitigating CO₂-caused climate change (e.g., globalwarming).

Moreover, the methanol product of reduction of carbon dioxide can beadvantageously used as (1) a convenient energy storage medium, whichallows convenient and safe storage and handling; (2) a readilytransported and dispensed fuel, including for methanol fuel cells; and(3) a feedstock for synthetic hydrocarbons and their products currentlyobtained from oil and gas resources, including polymers, biopolymers andeven proteins, which can be used for animal feed or human consumption.Importantly, the use of methanol as an energy storage and transportationmaterial eliminates many difficulties of using hydrogen for suchpurposes. The safety and versatility of methanol makes the disclosedreduction of carbon dioxide further desirable.

EXAMPLES

Embodiments of the invention are further explained by the followingexamples, which should not be construed by way of limiting the scope ofthe invention.

Example 1 General Electrochemical Methods

Chemicals and materials. All chemicals used were >98% purity and used asreceived from the vendor (e.g., Aldrich), without further purification.Either deionized or high purity water (Nanopure, Barnstead) was used toprepare the aqueous electrolyte solutions.

Electrochemical system. The electrochemical system was composed of astandard two-compartment electrolysis cell to separate the anode andcathode reactions. The compartments were separated by a porous glassfrit or other ion conducting bridge. 0.5 M KCl (EMD >99%) was used asthe supporting electrolyte. A concentration of the desired aromaticheterocyclic amine, such as pyridine, pyridine derivative, imidazole,imidazole derivative, of between about 1 mM to 1M was used.

The working electrode consisted of a known area Pt foil connected to aPt wire (both Aldrich) or a Pd foil (Johnson Matthey). Pd electrodeswere hydrogenated at a current density of 15 mA cm⁻² in 1 M H₂SO₄ until˜73 C were passed. All potentials were measured with respect to asaturated calomel reference electrode (SCE) (Accumet). Thethree-electrode assembly was completed with a Pt mesh electrode alsoconnected to a Pt wire. Before and during all electrolyses, CO₂ (Airgas)was continuously bubbled through the electrolyte to saturate thesolution. The resulting pH of the solution was maintained at about pH 3to pH 6, suitably, pH 4.7 to pH 5.6, depending on the aromaticheterocyclic amine employed. For example, under constant CO₂ bubbling,the pH's of 10 mM solutions of 4-hydroxy pyridine, pyridine, and4-tertbutyl pyridine were 4.7, 5.28 and 5.55, respectively. For NMRexperiments, isotopically enriched ¹⁵N pyridine (>98%) and ¹³C NaHCO₃(99%) were obtained from Cambridge Isotope Laboratories, Inc.

Example 2 General Photoelectrochemical Methods

Chemicals and materials. All chemicals used in this work were analyticalgrade or higher. Either deionized or high purity water (Nanopure,Barnstead) was used to prepare aqueous electrolyte solutions.

Photoelectrochemical system. The photoelectrochemical system wascomposed of a Pyrex three-necked flask containing 0.5 M KCl assupporting electrolyte and a 1 mM-1M catalyst, e.g., 10 mM pyridine orpyridine derivative. The photocathode was a single crystal p-typesemiconductor, which was etched for ˜1-2 min in a bath of concentratedHNO3:HCl, 2:1 v/v prior to use. An ohmic contact was made to the back ofthe freshly etched crystal using an indium/zinc (2 wt. % Zn) solder. Itwas then connected to an external lead with conducting silver epoxy(Epoxy Technology H31), that was covered in glass tubing, and insulatedusing an epoxy cement (Loctite 0151 Hysol) to expose only the front faceof the semiconductor to solution. All potentials were referenced againsta saturated calomel electrode (Accumet). The three-electrode assemblywas completed with a carbon rod counter electrode to minimize there-oxidation of reduced CO₂ products. During all electrolyses, CO₂(Airgas) was continuously bubbled through the electrolyte to saturatethe solution. The resulting pH of the solution was maintained at aboutpH 3-pH 6, e.g., pH 5.2.

Light sources. Four different light sources were used for theillumination of the p-type semiconductor electrode. For initialelectrolysis experiments, a Hg—Xe arc lamp (USHIO UXM 200H) was used ina lamp housing (PTI Model A-1010) and powered by a PTI LTS-200 powersupply. Similarly, a Xe arc lamp (USHIO UXL 151H) was used in the samehousing in conjunction with a PTI monochromator to illuminate electrodeat various specific wavelengths. A fiber optic spectrometer (OceanOptics S2000) or silicon photodetector (Newport 818-SL silicon detector)was used to measure the relative resulting power emitted through themonochromator. The flatband potential was obtained by measurements ofthe open circuit photovoltage during various irradiation intensitiesusing the 200 W Hg—Xe lamp (3 W/cm2-23 W/cm2). The photovoltage wasobserved to saturate at intensities above ˜6 W/cm2. For quantum yielddeterminations, electrolyses were performed under illumination by twodifferent light emitting diodes (LEDs). A blue LED (Luxeon V DentalBlue, Future Electronics) with a luminous output of 500 mW+/−50 mW at465 nm and a 20 nm fwhm was driven at its maximum rated current of 700mA using a Xitanium Driver (Advance Transformer Company). A Fraencollimating lens (Future Electronics) was used to direct the outputlight. The resultant power density that reached the window of thephotoelectrochemical cell was determined to be 42 mW/cm2, measured usinga Scientech 364 thermopile power meter and silicon photodector. It wasassumed that the measured power density is greater than the actual powerdensity observed at the semiconductor face since there is luminousintensity loss through the solution layer between the wall of thephotoelectrochemical cell and the electrode.

Example 3 Analysis of Products of Electrolysis

Electrochemical experiments were performed using a PAR 173potentiostat-galvanostat together with a PAR 379 digital coulometer, aPAR 273 potentiostat-galvanostat, or a DLK-60 electrochemical analyzer.Electrolyses were run under potentiostatic conditions from ˜6-30 hrsuntil a relatively similar amount of charge was passed for each run.

Gas Chromatography. The electrolysis samples were analyzed using a gaschromatograph (HP 5890 GC) equipped with an FID detector. Removal of thesupporting electrolyte salt was first achieved with Amberlite IRN-150ion exchange resin (cleaned prior to use to ensure no organic artifactsby stirring in a 0.1% v/v aqueous solution of Triton X-100, reduced(Aldrich), filtered and rinsed with a copious amount of water, andvacuum dried below the maximum temperature of the resin (˜60° C.) beforethe sample was directly injected into the GC which housed a DB-Waxcolumn (Agilent Technologies, 60 m, 1 μm film thickness.) Approximately1 g of resin was used to remove the salt from 1 mL of sample. Theinjector temperature was held at 200° C., the oven temperaturemaintained at 120° C., and the detector temperature at 200° C. During atypical run, only peaks related to the elution of methanol and pyridinewere observed.

Spectrophotometry: The presence of formaldehyde and formic acid was alsodetermined by the chromotropic acid assay. Briefly, a solution of 0.3 gof 4,5-dihydroxynaphthalene-2,7-disulfonic acid, disodium salt dihydrate(Aldrich) was dissolved in 10 mL deionized water before diluting to 100mL with concentrated sulfuric acid. For formaldehyde, an aliquot of 1.5mL was then added to 0.5 mL of sample. The presence of formaldehyde(absorbance at 577 nm) was detected against a standard curve using an HP8453 UV-Vis spectrometer. For formic acid, a 0.5 mL aliquot of samplewas first reduced with a ˜100 mg piece of Mg wire and 0.5 mLconcentrated hydrochloric acid (added slowly in aliquots over a 10 minperiod) to convert to formaldehyde before following the chromotropicacid assay as described above.

Mass spectrometry. Mass spectral data was also collected to identify allorganics. In a typical experiment, the sample was directly leaked intoan ultrahigh vacuum chamber and analyzed by an attached SRS Residual GasAnalyzer (with the ionizer operating at 70 eV and an emission current of1 mA). Samples were analyzed against standard methanol spectra obtainedat the same settings to ensure comparable fragmentation patterns. Massspectral data confirmed the presence of methanol and also proved thatthe initial solution before electrolysis contained no reduced CO₂species. Control experiments also showed that after over 24 hours underillumination the epoxy used to insulate the backside of the electrodedid not leach any organic material that would give false results for thereduction of CO₂ ¹⁷NMR spectra of electrolyte volumes after illuminationwere obtained using an automated Bruker Ultrashield™ 500 Plusspectrometer with an excitation sculpting pulse technique for watersuppression. Data processing was achieved using MestReNova software. Formethanol standards and electrolyte samples, the representative signalfor methanol was observed between 3.18-3.30 ppm.

NMR. NMR spectra of electrolyte volumes after bulk electrolyses werealso obtained using an automated Bruker Ultrashield™ 500 Plusspectrometer with an excitation sculpting pulse technique for watersuppression. Data processing was achieved using MestReNova software. Theconcentrations of formate and methanol present after bulk electrolyseswere determined using acetone as the internal standard. For ¹⁵N—¹³Ccoupling experiments, ¹³C NMR spectra were obtained using an automatedBruker Ultrashield™ 500 Plus spectrometer tuned for maximum ¹³Csensitivity. In a typical experiment, an aqueous solution containing 10%deuterium oxide (Cambridge Isotope Laboratories, Inc., >99.9%), 0.5MKCl, 50 mM of ¹⁵N pyridine, and 33 mM of ¹³C NaHCO₃ was first purgedwith Ar in a septa sealed NMR tube; then pH was adjusted using 1 M H₂SO₄to a pH of 5.2. At this pH, the bicarbonate was observed to becompletely in the dissolved ¹³CO₂ form. No peak associated with H¹³CO₃was seen. The temperature of the experiment was maintained at 295° K.,however, the temperature of the instrument was also adjusted to accessthe temperature dependence on the ¹⁵N—¹³C coupling. The sample tube washeld for at least 10 minutes at the given temperature before the spectrawere obtained to ensure temperature equilibrium. A temperature range of275° K. to 306° K. was examined.

Example 4 Electrolysis with Monosubstituted Pyridines

Electrolyses were performed for various monosubstituted pyridines. Theseelectrocatalysts were present at a concentration of 10 mM in 50 mL ofwater and 0.5 M KCl (as the supporting electrolyte). The cathode waseither Pt or hydrogenated Pd, galavanostatically held at 50 μA cm⁻², andsaturated with CO₂. Products of the electrolyses were analyzed asdescribed above in Example 3. The results, which are the averages of atleast three experiments, are given in Table 1 below.

TABLE 1 Results for bulk electrolyses for monoalkyl or amino-substitutedpyridine electrocatalysts Faradaic Yield (%) Electrocatalyst HCOOH^(a)CH₃OH Total Yield^(e) pyridinium 10.8 ± 0.5 22 ± 2 33 ± 3 2-methyl 16 ±4 26 ± 4 42 ± 8 pyridinium 4-methyl  7 ± 3 31 ± 3 38 ± 6 pyridinium4-tertbutyl trace 14.5 ± 2   14.5 ± 2   pyridinium 4-amino 12 ± 4 39 ± 451 ± 8 pyridinium 4-dimethyl  7 ± 2 11 ± 1 18 ± 3 amino pyridinium4-hydroxy 12 ± 1 15 ± 3 27 ± 4 pyridinum ^(a)Mostly in the formate format the pH's of the solutions used; ^(e)Total faradaic yield for observedCO₂-derived products, not including competing hydrogen generation.

Example 5 Electrolysis of Dialkylsubstituted Pyridines (Lutidines)

Electrolyses were performed for various disubstituted pyridines orlutidines. These electrocatalysts were present at a concentration of 10mM in 50 mL of water and 0.5 M KCl (as the supporting electrolyte). Thecathode was either Pt or hydrogenated Pd, galavanostatically held at 50μA cm⁻², and saturated with CO₂. Products of the electrolyses wereanalyzed a described above in Example 3. The results, which are theaverages of at least three experiments, are given in Table 2 below.

TABLE 2 Results for bulk electrolyses for various lutidineelectrocatalysts Faradaic efficiency η, % Electrocatalyst AcetaldehydeAcetone Methanol 2-propanol Ethanol 3,5-lutidinium trace trace-5.93.2-8.0  trace-9.5 trace-5.3 3,4-lutidinium trace trace-5.6 4.4-9.2trace-12 trace-3.0 2,3-lutidinium trace trace-9.4 5.9-8.4 trace-12trace-3.4 2,4-lutidinium trace trace-8.6 4.5-13  trace-16 trace-5.42.6-lutidinium trace 2.9-5.8 3.6-8.2 trace-14 trace-6.3

Example 6 Electrolysis of 4,4′-bipyridine

Electrolysis was carried out in aqueous solution containing 10 mM4,4′-bipyridine with 0.5 M KCl as the supporting electrolyte. The pH wasmaintained at a constant 5.22, under CO₂ saturation. The resultsrepresent the average of at least three independent experiments, and aregiven below in Table 3.

TABLE 3 Electrolysis of 4,4′-bipyridine Faradaic efficiency η, % E (V)aAcetaldehyde Acetone Methanol 2-Propanol Ethanol −0.725 trace 3 ± 1 53 ±13  4 ± 2 trace −1.1 trace 7 ± 4 20 ± 2  20 ± 8 trace

Example 7 Photoelectrolysis of Lutidines and Bipyridine

Photoelectrolysis was carried out in aqueous solution containing 10 mMlutidine or bipyridine electrocatalyst in 50 mL water with 0.5 M KCl asthe supporting electrolyte. The pH was maintained at a constant 4.7,under CO₂ saturation. The photoelectrochemical cell was a p-GaP,p-GaInP2, or p-Si photoelectrode. For the p-GaP system, the wavelengthof 365 nm was chosen to correspond to the lowest energy direct bandgapof 2.8 eV. With an indirect bandgap of 2.24 eV, p-GaP can only absorb˜17% of solar radiation. Therefore, both a p-GaInP2 photocathode, with adirect bandgap of 1.81 eV, and p-Si, with an indirect bandgap of 1.12eV, were examined as potential photoelectrodes to increase thepercentage of solar radiation that can be converted into stored chemicalenergy. The commercially available wavelengths of 465 nm and 530 nm wereexamined for illumination of the p-GaInP₂ cell and the p-Si cell,respectively. However, wavelengths as long as ˜685 nm and ˜1100 nm canbe used to excite p-GaInP₂ and p-Si, respectively.

At a pH of 5.22 for the 4,4′-bipyridine system, the thermodynamicpotential for the reduction of CO₂ to methanol is approximately −0.52 Vvs. SCE. At the pH of 5.55 for the lutidine-based system, this potentialis −0.54 V vs. SCE. Similarly, the thermodynamic reduction potential ofCO₂ to 2-propanol at the pH's of 5.22 and 5.55 is −0.52 V vs. SCE and−0.54 V vs. SCE, respectively. The results of bulk electrolysis given asfaradaic efficiencies are reported below in Table 4.

TABLE 4.4 Faradaic efficiencies and quantum yields for the reduction ofCO₂ to various products. E(V) Faradaic Efficiency η, (%) vs. j(μA/ Ace-2- Cat. SCE cm²) Acetal tone MeOH PrOH EtOH p- 4,4′ −0.5 241 Trace trace5.5 1.5 — GaInP₂ 4,4′ −0.4 181 Trace trace 8.2 2.8 — 465 nm 4,4′ −0.3 70Trace trace 29 22 — 3,5 lut^(a) −0.5 146 Trace 5.9 7.2 3.7 — 2,3 lut−0.5 8.1 Trace trace 9.2 5.3 — 2,4 lut −0.5 161 Trace trace 21 8.3 — 2,6lut −0.5 14 Trace trace 51 21 — p-GaP 3.5 lut −0.5 10 Trace 2.9 51 trace— 365 nm 3,4 lut −0.5 13 Trace trace 63 15 — 2,3 lut −0.5 42 Trace trace48 28 — 2,4 lut −0.5 32 Trace trace 21 29 — 2.6 lut −0.5 9.5 Trace trace38 63 — 4,4′ −0.4 70 Trace trace 11 5 — p-Si 4,4′ −0.6 41.5 Trace trace18 24 — 530 nm ^(a)“lut” is a an abbreviation for lutidine; 4,4′ is anabbreviation for 4,4′-bipyridine.

Enhanced yields were observed for the lutidinium catalysts and highyields for 4,4′-bipyridinium. In some instances, nearly 100% faradaicyields were observed for CO₂-derived products. The yields for 2-propanolrepresent the highest reported 2-propanol yields known to the inventorswith 2,6-lutidine in a p-GaP photoelectrochemical system yielding ashigh as 63% faradaic efficiency for 2-propanol. The data reported inTable 4 for methanol and 2-propanol, at −0.5 V vs. SCE and potentialsless negative, was observed at essentially zero overpotential, that is,at a potential approaching the short circuit potential for the iRcompensated cell. This corresponds to the conversion of light energyinto storable chemical energy in the form of highly reduced CO₂products. To the inventors' best knowledge, this is the first report ofthe reduction of CO₂ to 2-propanol using only energy.

Example 9 Electrolysis/Photoelectrolysis of Imidazoles

Electrolyses are performed for various imidazoles, includingbenzimidazoles. These electrocatalysts are present at a concentration of10 mM in 50 mL of water and 0.5 M KCl (as the supporting electrolyte).The cathode is Pt or illuminated P—GaP, galavanostatically held at 50 μAcm⁻², and saturated with CO₂. Products of the electrolyses are analyzedas described above in Example 3.

Example 10 Electrolysis/Photoelectrolysis at Varying Temperatures andPressures

Electrolyses are carried out at various temperatures and pressures todetermine their effect on the products and yield. Electrolyses arecarried out at temperatures from 10° C. to 50° C. The resultsdemonstrate that the kinetics of the electrolysis reaction are increasedwith increased temperature, with peak currents following the Arrheniusrate law with an observed activation barrier of 10-15 kJ/mole, althoughthe solubility of the CO₂ is decreased with increased temperature.

Electrolyses are carried out at pressure from 1 atmosphere to 10atmospheres. The results demonstrate that product yields can beincreased with increased pressure due to the increased solubility of CO₂at higher pressures. For example, pressure data at 5 atm indicates a 3.5increase in current compared to 1 atm.

Temperature and pressure can be optimized to produce efficient andhigher product yields.

In summary, embodiments of the invention provide that carbon dioxide canbe efficiently converted to value added products, using either a minimumof electricity (that could be generated from an alternate energy source)or directly using visible light. The processes of the embodiments of theinvention generate high energy density fuels that are not fossil-basedas well as being chemical feedstocks that are not fossil or biologicallybased. Moreover, the catalysts for these processes aresubstituents-sensitive, and provide for selectivity of the value addedproducts.

The foregoing description is considered as illustrative only of theprinciples manifest in embodiments of the invention. Numerousmodifications and changes may readily occur to those skilled in the art,and it is not desired to limit the invention to the exact constructionand operation shown and described, and accordingly, all suitablemodifications and equivalents are considered to fall within the scope ofthe invention. Various features and advantages of the invention are setforth in the appended claims and their equivalents. It is intended thatthe scope of the present invention be limited solely by the broadestinterpretation that lawfully can be accorded the appended claims.

All publications, patents and patent applications referenced in thisspecification are indicative of the level of ordinary skill in the artto which this invention pertains. All publications, patents and patentapplications are herein expressly incorporated by reference to the sameextent as if each individual publication or patent application wasspecifically and individually indicated by reference. In case ofconflict between the present disclosure and the incorporated patents,publications and references, the present disclosure should control.

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1. A method of converting carbon dioxide to provide at least one productselected from the group consisting of glyoxal, isopropanol, ethanol,2-propanol, acetone, acetaldehyde and mixtures thereof, comprisingreducing the carbon dioxide electrochemically in a dividedelectrochemical cell comprising an anode in one cell compartment and acathode in another cell compartment, the cathode selected from the groupconsisting of Au, Ag, Zn, Ga, Hg, In, Cd, Ti, Pt and hydrogenated Pd,the cell compartment including the cathode including one or moresubstituted or unsubstituted aromatic heterocyclic amines, eachcompartment containing an aqueous solution of an electrolyte, whereinsaid at least one product selected from the group consisting of glyoxal,isopropanol, ethanol, 2-propanol, acetone, acetaldehyde and mixturesthereof is generated in the electrolyte by reduction of carbon dioxideand the aromatic heterocyclic amine is selected from the groupconsisting of lutidines and bipyridines, and mixtures thereof.
 2. Themethod of claim 1 wherein the cathode is selected from the groupconsisting of Pt and hydrogenated Pd.
 3. The method of claim 1, whereinthe lutidines and bipyridines are selected from the group consisting of2,6 lutidine and 4,4′ bypryridine.
 4. A method of converting carbondioxide to provide at least one product selected from the groupconsisting of formic acid, glyoxal, isopropanol, ethanol, 2-propanol,acetone, acetaldehyde and mixtures thereof, comprising reducing thecarbon dioxide electrochemically in a divided electrochemical cellcomprising an anode in one cell compartment and a cathode in anothercell compartment, the cathode selected from the group consisting of Au,Ag, Zn, Ga, Hg, In, Cd, Ti, Pt and hydrogenated Pd, the cell compartmentincluding the cathode including one or more substituted or unsubstitutedaromatic heterocyclic amines, each compartment containing an aqueoussolution of an electrolyte, wherein the aromatic heterocyclic amine is alower alkyl substituted pyridine, a lower alkyl amino substitutedpyridine, an unsubstituted bipyridine, a lower alkyl substitutedbipyridine, hydroxy-pyridine, hydroxy-bipyridine, or lower alkyl aminosubstituted bipyridine.
 5. A method of converting carbon dioxide toprovide at least one product selected from the group consisting offormic acid, glyoxal, isopropanol, ethanol, 2-propanol, acetone,acetaldehyde and mixtures thereof, comprising reducing the carbondioxide electrochemically in a divided electrochemical cell comprisingan anode in one cell compartment and a cathode in another cellcompartment, the cathode selected from the group consisting of Au, Ag,Zn, Ga, Hg, In, Cd, Ti, Pt and hydrogenated Pd, the cell compartmentincluding the cathode including one or more substituted or unsubstitutedaromatic heterocyclic amines, each compartment containing an aqueoussolution of an electrolyte, wherein the aromatic heterocyclic amine is apyrazine, a pyridazine or a pyrimidine represented by formula 3:

wherein one of L1, L2 or L3 is N and the other L's are C, R9 is H, if L1is N, then R10 is H, if L2 is N, then R11 is H, and if L3 is N, then R12is H; and if L1, L2 or L3 is C, then R10, R11, R12, R13 and R14 areindependently selected from straight chain or branched chain loweralkyl, hydroxyl, amino, or pyridyl.
 6. A method of converting carbondioxide to provide at least one product selected from the groupconsisting of formic acid, glyoxal, isopropanol, ethanol, 2-propanol,acetone, acetaldehyde and mixtures thereof, comprising reducing thecarbon dioxide electrochemically in a divided electrochemical cellcomprising an anode in one cell compartment and a cathode in anothercell compartment, the cathode selected from the group consisting of Au,Ag, Zn, Ga, Hg, In, Cd, Ti, Pt and hydrogenated Pd, the cell compartmentincluding the cathode including one or more substituted or unsubstitutedaromatic heterocyclic amines, each compartment containing an aqueoussolution of an electrolyte, wherein the aromatic heterocyclic amine isrepresented by formula 4:

wherein L⁵ is N or C, R¹⁵ is H, R¹⁶ is H if L⁵ is N, or if L⁵ is C, R¹⁶,R¹⁷, R¹⁸ and R¹⁹ are independently selected from straight chain orbranched chain lower alkyl, hydroxyl, amino, or pyridyl, or R¹⁷ and R¹⁸taken together with the ring carbons to which they are bonded are afused 5-member aryl ring.
 7. The method of claim 6, wherein the aromaticheterocyclic amine is an imidazole or a pyrrole.
 8. The method of claim1, which further comprises obtaining the carbon dioxide from an exhauststream from a fossil fuel burning power or industrial plant, from asource accompanying natural gas or from a geothermal well.
 9. The methodof claim 1, wherein electrical energy for the electrochemical reductionof carbon dioxide is provided from an energy source based on nuclear,hydroelectric, wind, geothermal or solar power.
 10. A method ofconverting carbon dioxide to provide at least one product selected fromthe group consisting of formic acid, formaldehyde, glyoxal, methanol,isopropanol, ethanol, 2-propanol, acetone, acetaldehyde and mixturesthereof, comprising reducing the carbon dioxide electrochemically in adivided electrochemical cell comprising an anode in one cell compartmentand a cathode in another cell compartment, the cathode selected from thegroup consisting of Au, Ag, Zn, Ga, Hg, In, Cd, Ti, Pt and hydrogenatedPd, the cell compartment including the cathode including one or moresubstituted or unsubstituted aromatic heterocyclic amines exceptpyridine, p-aminopyridine, p-ethylpyridine and 4-methyl quinoline, eachcompartment containing an aqueous solution of an electrolyte, whereinthe aromatic heterocyclic amine is a pyrazine, a pyridazine or apyrimidine represented by formula 3:

wherein one of L1, L2 or L3 is N and the other L's are C, R9 is H, if L1is N, then R10 is H, if L2 is N, then R11 is H, and if L3 is N, then R12is H; and if L1, L2 or L3 is C, then R10, R11, R12, R13 and R14 areindependently selected from straight chain or branched chain loweralkyl, hydroxyl, amino, or pyridyl.